[codex] Cover stale post_parent semantic references

[adamziel]

What it does

Adds a focused WordPress semantic validator regression for wp_posts.post_parent references. The fixture covers a source branch deleting a parent post while the target edits child page and attachment rows that still point at that parent.

Rationale

post_parent is a WordPress object edge, not just a scalar column. A row-level merge can legally apply the parent delete and keep target child edits, but the merged state is incoherent unless a semantic validator reports the stale parent reference for review.

Implementation

The new discovered validator scans child posts with post_parent > 0, checks that the parent row still exists, and emits plugin-wp-post-parent-missing-reference findings with the child post ID, post type, field name, and missing parent ID. The test asserts that the parent delete remains staged, target child edits remain visible, and the audit output exposes both the page and attachment children.

The merge reliability matrix now lists post_parent references in the covered WordPress semantic validator evidence.

Testing instructions

php -l tests/cow/wp_semantic_validator.php
make test-cow-wp-semantic-validator
make test-cow-semantic-fast
npm run check
git diff --check